Polymer particles of controlled sizes are desirable in a wide variety of applications. In the imaging industry, they can be used to prepare beads that incorporated in the top most layer of the imaging element. The size of these beads determines the gloss imparted to the image. They can also be used in ink formulations, particularly aqueous, where upon drying they form a film to encapsulate the pigments or dyes. In electrophotography, polymeric particles containing pigments are used as toners or dry inks. While it is well known in the art to prepare vinyl or addition type polymer particles of varying sizes by emulsion or suspension polymerization techniques, it is much more difficult to prepare particles comprising condensation type polymers. Even in the case of vinyl polymers, it is not easy to prepare particles where there are mixtures of more than one polymer. Typically, in these instances insitu polymerization techniques such as those mentioned above will result in copolymers being formed rather than physical mixtures of polymers.
In these instances an emulsion technique can be used, wherein an organic phase comprising the desired mixture of polymers is dissolved in a suitable volatile solvent, which is then emulsified with an aqueous medium containing appropriate stabilizers to form an oil-in-water type of emulsion. The solvent is then removed from the oil-in-water emulsion by evaporation resulting in a product comprising an aqueous slurry of polymer particles in an aqueous medium. The evaporative method of making polymer particles is more versatile than the insitu polymerization techniques, because of the reasons mentioned above.
The method of removing the solvent from the oil in water emulsion is one major factor in the cost of producing the polymer particles. This is particularly important for cost sensitive products such as toners for electrophotographic printing and matting agents. Most of the methods described in the prior art relate to batch processes. U.S. Pat. No. 4,833,060 describes using a Nitrogen stream passing through the vapor space above a stirred emulsion, thereby carrying away vapors of the solvent. This method can increase the cost of the process not only because of the time involved, but because of the cost of the nitrogen that is used. Nitrogen is desired from a safety point of view, because of the flammability of most practical types of volatile solvent. Furthermore, the cost of recovering the solvent vapors in a diluted stream is also high—solvent recovery is desirable from both a solvent cost as well as an environmental point of view. U.S. Pat. No. 6,380,297 describes a vacuum evaporation method, where the lower pressure enables the removal of solvent at a lower temperature. While this method has the potential for lower cost than the nitrogen sweep method, it also has its problems. The time required to remove the solvent from the emulsion increases in proportion to the size of the batch. This can create a bottleneck in the manufacturing process, thereby increasing the size of the equipment that need to be used. Additionally, the longer time required by the emulsion to remain at an elevated temperature, contributes to coalescence occurring in the emulsion and change in the particle size distribution of the process. Thus, it is desirable to remove the solvent from the process in a continuous manner.
U.S. Pat. No. 5,580,692 discloses the use of water to extract the solvent from the emulsion. This technique can be used when the volatile solvent has some miscibility in water. However, the amount of water required to extract the solvent is large and subsequent recovery of the solvent from the water-solvent mixture can be quite difficult.
There are several processes known to those skilled in the art to continuously evaporate a solvent from a fluid or suspension. Flash evaporators remove the solvent by raising the temperature of a fluid to a desired temperature and then flashing it into a low pressure environment, such that the fluid temperature is substantially higher than the boiling point of the solvent at the lowered temperature. Thus, the solvent is separated under adiabatic conditions—no heat is provided during the boiling stage. While this method can achieve continuous removal of solvent in a relatively short period of time, the ability to decrease the residual solvent in the polymer particles to a desired low level becomes increasingly difficult as the solvent level in the initial emulsion becomes higher. This is because under adiabatic conditions, the higher the amount of solvent removed the greater is the degree of cooling, from the latent heat of vaporization and eventually at a high solvent level, the product will cool to below the boiling point of the solvent, resulting in high residual levels of solvent in the product.
Another continuous method is the falling film evaporation, where the emulsion falls down the side of a tube of a heat exchanger to which heat is provided on the other side of the tube by a heating fluid. U.S. Pat. No. 4,833,060 discloses the use of a falling film evaporator to remove the solvent from the emulsion. The process can also be carried out under reduced pressure, just as the flash evaporation method was carried out. In this instance, the solvent removal occurs under isothermal conditions, and thus solvent levels in the emulsion are not restricted. However, the heat transfer needed to provide boiling is dependent on the thickness of the film, which in turn is determined by the gravity force. Thus, this process is not flexible in controlling the heat transfer coefficient, and subsequently results in relatively large equipment size.
Among all previous mentioned methods of solvent removal the falling film evaporator is the most amenable to the removal of solvent for this application. However it is desirable to control the film thickness to optimize the heat transfer rates in order to achieve the desired product characteristics with the smallest equipment possible. The spinning disc reactor is an attempt to apply the principles of process intensification to the fields of heat and mass transfer. This type of reactor includes within a chamber a plate like member or an assembly of a plurality of such members which is rotated about its central axis, usually a vertical axis but a horizontal axis or any other orientation is not excluded, to effect transfer of a liquid material from the central axis radially outward under the influence of centrifugal force and in the form of thin often wavy films across the plate or plates. Such thin films have been shown to significantly improve the heat and mass transfer rates and mixing. The technology was developed for various common heat and mass transfer and mixing applications such as disclosed in R J J Jachuck and C Ramshaw, “Process Intensification: Heat transfer characteristics of tailored rotating surfaces”, Heat Recovery Systems and CHP, Vol. 14, No 5, pp. 475-491, 1994. The properties of these thin often wavy films can be manipulated by changing the disc rotational speed, the throughput of the materials fed to the disc, the temperature of the disc and the configuration of the disc surface.
EP 0499 363 describes the utilization of a spinning disc reactor for the photo-catalytic degradation of organic materials.
EP 1152 823 describes the utilization of a spinning disc for polymerization of ethylene on a catalyst coated disc, precipitation of barium sulphate crystals and precipitation of calcium carbonate crystals.
US Publication 2004/0235039 describes the utilization of a spinning disc reactor for the free radical polymerization of styrene and the cationic polymerization of styrene.
WO2003/008460 A1 further describes methods of polymerizing chemical components using a spinning disc reactor.
US Publication 2004/0241430 and WO 2003/008 083, further describe the utilization of a spinning disc reactor for the precipitation of small particles.
WO 0004/8728A1 describes the benefits of spinning disc reactors with enhanced surface features, seemingly similar to EP 1169 125 B1 which describes a “rotating surface of revolution reactor with shearing mechanisms.
U.S. Pat. No. 6,515,153 and US 2002/0035281A1 describe the utilization of a spinning disc reactor for the forming of amido esters.
W0021 8328 A1 describes the utilization of a spinning disc reactor for reacting carboxylic acids and esters.
EP 146 4389 describes an embodiment of a spinning disc which includes a rotary fan or impeller to remove a gaseous component from a region surrounding the periphery of the surface.
U.S. Pat. No. 6,858,189 B1 and US Publication 2003/0161767 A1 describe various feeding and collection mechanisms suitable for a spinning disc reactor.
The spinning disc has attractive properties beneficial in the current invention including intense mixing in the thin liquid film, short residence times, plugflow characteristics, easy cleaning, high solid/liquid heat/mass transfer rates, high liquid/vapor heat/mass transfer and high energy efficiency.
The prior art does not teach the removal of solvents utilizing a spinning disc reactor and furthermore does not teach the removal of solvents from within particles dispersed in an aqueous medium.